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Old 23-02-2007, 13:16   #1
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One Solar Charge Controller or Two?

I am putting together my solar charging system at the moment. I have two Kyocera 125 Watt panels and plan on adding two more. Two will be mounted on foldable stanchion mounts and two will be mounted on the cabin top of my Prout.

At this point I am planning to series two panels and run them into an MPPT controller. I will add the two additional panels later. If I go with one charge controller I will need to get a larger model, such as the Outback MX60 or Blue Sky SB50, to support the maximum current. Another option I was considering was to buy a 25 amp controller such as a Blue Sky SB2000. As I add another set of panels I could add a second controller and wire the outputs in parallel to the batteries.

Pros and cons...

One controller:
One piece of equipment to mount and one set of wires to the batteries. (not a big deal in my case).
Perhaps a little bit cheaper than two smaller controllers (but not much).
No concern about the controllers fighting each other**.

No redundancy or backup. Loose the controller and loose all of the solar power.
Less efficiency: If the cells are partially shaded then the reduced current from one panel will drag down the other in series. This in turn could affect the MPPT unit to adjust the panels in parallel and reduce the overall efficiency.

Two Controllers:
Equipment redundancy. Even if I loose one controller I would still have half of my panels working for me.
Each controller can adjust its MPPT point to the highest efficiency for its panels.
I can mix panels sizes and types since the 125W panels have been replaced with 130W panels. I could put my two current panels on one controller and place the new panels on their new controller so each MPPT controller could adjust separately.

More Equipment and more wires to mount. (Not too big of a deal)
Slightly higher cost.
The charge controller outputs could interfere with each other**. (I am a little fuzzy on this).

**Am I on the right track here? If I have two controllers feeding the same 550 AH battery bank could one be in the bulk charge mode and cause the other to think it should be in float mode?

Any feedback will be helpful.

Thanks, Woody

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Old 23-02-2007, 13:50   #2

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Get one big controller. A good quality piece, properly installed and kept dry should work for a very long time.
You're right about two interfering. One is likely to sit back to watch the show while the other toils.
As for mixing panels, as long as you install similar panels in series to be paralleled by others with approximately the same voltage outputs, you should be fine. IOW, 2 120's in series paralleled by 2 100's in series, paralleled by 2 75's in series etc. would be ok.

Steve B.

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Old 23-02-2007, 14:12   #3
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One controller but

Makai carries 4 kyrocea 120's on one sb50 controller. The max peak is about 34-36 amps of which the 50 will do 50 but can handle up to 60 peak according to the documentation.

If you really want 2 controllers BlueSky makes 25-30 amp controllers that can be ganged with one as the primary controller so you don't get the fighting or the unbalance. This still provides redundency if that is what you want while still maximizing the panels.

We have been using our sb50 for over 3 years and it works perfect. As suggested above. WIre and keep it dry and the quality of the bluesky will eliminate the need for an expensive second unit.

Lastly if you wanted back up buy a cheap PWN like the morningstar and toss it in the spares box for just in case
Captain Bil formerly of sv Makai -- KI4TMM
The hunt for the next boat begins.
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Old 04-03-2007, 22:30   #4
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More than one DC charge controller?

I too have two 120 watt panels, with a plan for two more as I expect will be necessary soon. I too am debating one or two controllers. I am also trying to determine if its even possible for two or more controllers to work effectively together, as when one senses a low battery voltage and starts charging, the other will sense the charging voltage of the first and stop charging. I have a Rutland 913 wind generator that also has a controller and I haven't finished the physical install (on the mizzen) to get to the electrical part - but I still haven't bought a solar controller, as I don't know if there is any that will also serve my wind genny (when really windy it can put out a very high voltage which can damage some controllers (Solar Boost 50 for one)). To complicate matters, I have also installed a pulley on my prop-shaft (after years of research and planning), and hope to belt an alternator off it for another continuous charge while sailing, which I would also like to run through the DC charge controller. The question of how many charge controllers to have is troublesome, as I still don't get how more than 1 will operate at the same time, although there seem to be lots of boats with multiple charging sources...
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Old 04-03-2007, 22:40   #5
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The main problem with MPPT contollers is that none of them seem designed for marine use, ie, they are not conformal coated and have a very low IP rating. (Things may have changed but my research is only 18 months old) Figure the small efficiency loss through a conventional (marine) regulator into your equation but whatever you do mount the thing as far away from the ocean as possible.
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Old 04-03-2007, 23:27   #6
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Candine could you give more information on your prop shaft alternator drive ? This is somthing that l have contemplated for a long while too. Did you add any extra bearings to take the side load ?
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Old 04-03-2007, 23:29   #7
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Why have the solar panels in series? Do you have a 24 volt system? Or is there something about MPPT that increases efficiency with a larger difference in input to output voltage?

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Old 05-03-2007, 02:11   #8
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With an MPPT controller. having the panels in series will be an advantage if the panels have per-cell bypass diodes (which are different than the isolation diodes most panels have).

If a cell gets shaded in a series-connected panel bank, that panel will still deliver current from the non-shaded cells, but the panel voltage will be lower. The MPPT controller can still convert the panel output to the appropriate battery charging voltage.

If a cell gets shaded in a parallel-connected panel bank, that panel will have a lower output voltage. The higher voltage of the unshaded panel will effectively reverse-bias the shaded panel, which will then deliver less power then it otherwise could.

This series connection works (when there are bypass diodes) up to the point where the output voltage of the series-connected bank is greater than the MPPT controller can handle. In the 12V-system MPPT controller I've seen the specs for, two series-connected panels would be OK, but three panels would exceed the controller spec.

I am still trying to figure out if my panels have the bypass diodes. I know how to test it, but I would like to find a specification first.
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
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Old 05-03-2007, 04:10   #9
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What ga wire are you using in your solar system?

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